1. Field of the Invention
The present invention relates to developing devices, process cartridges and image-forming apparatuses, more particularly, conveyance and stirring mechanisms of two-component developing agents.
2. Description of Related Art
In image-forming apparatuses such as copying machines, printers, facsimile apparatuses and printing machines, an electrostatic image on a photoconductor as an image carrier is visualized with a developing apparatus and is transferred onto a sheet and so on to obtain a record-output.
For the developing agent used in the developing, there is a two-component-type composed of toner and carrier mixture, in addition to a one-component type only of magnetic or non-magnetic toner.
The two-component developing agent is composed of toner and its carrier, and by charging the toner with triboelectric charging when mixing by stirring, the toner becomes in a condition able to adhere electro-statically onto a latent electrostatic image on a photoconductor.
As a configuration of the developing apparatuses to mix the developing agent in the course of a conveying process, it is proposed that screws used in conveying the developing agent be arranged one above the other, and supply and recovery units of the developing agent be arranged vertically relative to a developing agent-bearing member (for example, see Patent literatures 1-3). Patent literature 1 is JPA No. 1111-174810, Patent literature 2 is JPA No. H05-333691 and Patent literature 1 is JPA No. 2003-307924.
Patent literature 1 discloses a configuration in which the developer supply unit is arranged under the developer recovery unit, and discloses a blade to control the layer thickness of the developing agent and its arrangement under the developing agent-bearing member on the side of the developer supply unit.
Patent literatures 2 and 3 disclose another configuration regarding the positional relationship; namely, the developer supply unit and the developer recovery unit of Patent literature 1 are changed in a vertical direction.
In the developer recovery unit, the developing agent increases in level by going from upstream to downstream due to a rotation of the conveying screw. In sum, FIG. 11 shows a configuration as disclosed in Patent literature 1.
As shown in FIG. 11, a developing agent is conveyed and mixed by a conveyance screw 101 provided in a conveyance space GR1 in the recovery side, and once the developing agent has reached an end of the conveyance screw 101, this developing agent surges to the side of a communication section R1 which leads to a further increase in the level due to its accumulation.
In the developer supply unit as shown in FIG. 11, the volume of the developing agent decreases by toner consumption of the developing agent due to its adhesion to a developing roller in the course of conveying by a conveyance screw 102 provided in a conveyance space GR2. So, the volume of the developing agent decreases by following through to the end of the conveyance screw 102, which leads to a decrease in the level of developing agent.
The developing agent reaches to the end of the conveyance screw 102, and moves through a communication section R2 to the conveyance space GR1 again, so that one-way circulation of the developing agent is accomplished.
As for such a one-way circulation of the developing agent described above, it is necessary that the circulation of the developing agent be carried out steadily, but if the level of the developing agent is increased in the developer recovery unit as above-mentioned, it possibly becomes difficult to recover the developing agent.
As a result, it becomes impossible to recover from a developing agent-bearing member. If this member is a rolling member such as a roller and so on, one portion of the unrecovered developing agent is carried together with the rotation of the rolling member, and it sometimes invades directly into the developer supply unit.
This developing agent which directly invades into the developer supply unit from the developing roller is not one with the triboelectric charging by mixing being carried out properly, so that it possibly leads to a concentration deviation if it is supplied into the developing roller.
To increase the recovery efficiency, it is considered to increase a rotation speed of the conveyance screw. However, there is a possibility of causing toner degradation in the developing agent, due to friction heat caused by friction of the developing agent in the position where the level of the developing agent is increased.
In addition, it is necessary to increase a driving power in accordance with an increase in load on the conveyance screws due to the increased level of the developing agent, which results in the electric consumption of the driving source being increased.